Plasma Enhanced Chemical Vapor Deposition (PECVD) methods for coating external surfaces of a workpiece within a vacuum chamber are well known. The coating of interior surfaces of hollow workpieces, such as pipes, using PECVD techniques is less common, but has been described in U.S. Pat. No. 7,300,684 to Boardman et al., which utilizes a high deposition rate PECVD technique. The Boardman et al. method involves using the workpiece itself as a vacuum chamber, coupling a gas supply to one opening and a vacuum pump to another, and employing a voltage biasing system in which the negative terminal is attached to the pipe and the return anodes are located near the ends of the pipe but isolated from the pipe. The gas supply provides hydrocarbon precursors and the voltage biasing system is used to generate a high density hollow cathode plasma and to attract hydrocarbon ions to the surface, so as to form a diamond-like carbon (DLC) film on the interior surface of the pipe. Alternatively, non-hydrocarbon precursors can be utilized to form coatings other than DLC.
As used herein, the term “hollow cathode effect” is an occurrence such as in a tubular geometry with an axial anode, in which at least two cathode surfaces are positioned facing each other with a space between the cathode surfaces and the anode, and biasing and pressure parameters are such that a large increase in current is achieved as compared to a conventional plasma glow. Such cathode surfaces can be coaxial internal wall surfaces in a pipe. The increase in current is due to the “oscillation motion” of fast (hot, accelerated) electrons between the opposite space charge sheaths, which enhances the excitation and ionization rates in the plasma several orders higher than in the conventional glow discharge. The following definitions and descriptions of the hollow cathode effect are contained in the publication entitled “STUDIES OF HOLLOW CATHODE DISCHARGES USING MASS SPECTOMETRY AND ELECTROSTATIC PROBE TECHNIQUES” by H. S. Maciel et al., 12th International Congress on Plasma Physics, 25-29 Oct. 2004, Nice (France). Hollow cathode discharges are capable of generating dense plasma and have been used for development of high-rate, low-pressure, high-efficiency processing machines. The geometric feature of a hollow cathode discharge promotes oscillations of hot electrons inside the cathode, thereby enhancing ionization, ion bombardment of inner walls, and other subsequent processes. At the same time, the hollow cathode exhibits plasma density one to two orders of magnitude higher than that of conventional planar electrodes. “It is well known that the product (Pd), of the inter-cathode distance (d) by the pressure (P), is an important parameter to describe the behavior of the HC discharge. Usually, the electron-atom inelastic collision rates are increased by the decrease of the inter-cathode distance with a large effect on the plasma density and electron temperature. The effect of the gas pressure on the discharge properties is expected since the increase in the collisionality by increasing the pressure tends to enhance the hollow cathode effects being possible to reach an optimized reduced inter-cathode distance (Pd).”
The system described in the Boardman et al. patent operates well for its intended purpose. However, for relatively long and high aspect ratio passageways, there are potential difficulties with maintaining plasma uniformity down the full axial length. As used herein, the “aspect ratio” of a passageway within a pipe or other workpiece is defined as the ratio of the length of the passageway to the cross sectional dimension (typically, a diameter) of the passageway. In conventional approaches, a pipe or other tubular workpiece may be placed for external coating in a chamber in which dimensions are designed such that there is little change in pressure throughout the chamber. However, when using the interior of the workpiece as the chamber, the dimensions of the chamber are defined by the intrinsic internal dimensions of the workpiece. For high aspect ratio workpieces in which the hollow cathode effect is utilized, there is a weak plasma within the central region of the interior passageway of the workpiece, while the ends of the passageway have an intense plasma. One possible explanation is that a high impedance is encountered by electrons leaving the center of the workpiece (which is biased as the cathode) while a lower impedance is encountered by electrons leaving the ends. As a result, electron current is shunted to the ends of the workpiece.
One possible solution is described in US Publication No. 2006/0196419 to Tudhope et al., which is assigned to the assignee of the present invention. As described in this reference, the interior surface of a workpiece can be coated in sections. Rather than having anodes attached at the opposite ends of the workpiece, a pair of anodes are located within the workpiece at a distance from each other and are systematically moved along the length of the workpiece. Thus, while the aspect ratio of the workpiece is not controllable, the aspect ratio of the section being coated is controlled. Another method relevant to the Boardman et al. patent is described in US Publication No. 2006/0198965. Rather than a continuous flow in one direction, the flow of gas is systematically reversed for the purpose of providing a more uniform coating along the interior surface of the workpiece.
While the use of the hollow cathode effect is not described, other approaches that are of interest are described in U.K. Patent Application No. 2030180 A to Sheward. In one embodiment described in Sheward, a positively biased anode extends along the length of the interior passageway of a tube being internally coated. In an alternative embodiment, the solid anode is replaced with an anode having a series of holes through which relevant gas is released.
A concern with placing an anode wire along the axis of a high aspect ratio passageway is that while a plasma may be maintained, the hollow cathode effect is easily lost and the deposition rate is lowered. Moreover, as the plasma impedance down a long workpiece can vary for many different reasons, including differences in pressure, gas composition, distance between the electrodes, and incidental coating of the anode wire, plasma intensity is further reduced and/or “hot spots” develop as plasma concentrates at one or more still high conductivity regions of the anode.
Tubular structures with coated interior carrier surfaces are described in U.S. Pat. No. 7,351,480 to Wei et al. Plasma immersion ion processing for coating of hollow substrates is described in US Publication No. 2008/0292806 A1.
Further improvements to the coating of high aspect ratio passageways are sought.